Various methods have been utilized to treat a waste solution containing metallic ions. Ion exchange, membrane separation, solvent extraction, electrowining and precipitation are typical treatments for a waste solution containing a low concentration of metallic ions. As to the treatment of a waste solution containing a high concentration of metallic ions such as an electroplating waste solution and a thick leach solution of metal scrap, a complicated process involving several of those treatments is generally required, and thus the operation cost thereof is relatively high. When the waste solution contains different metallic ions of high concentrations, the separations of metallic ions from the waste solution become more difficult. Therefore, an ideal process for treating the waste solutions should be able to minimize the steps of separations, and preferably recycle useful products therefrom.
Upon electroplating a single metal such as zinc and nickel metal, impurity control in the electroplating bath is severe C. Cachet, R. Wiart, "Zinc deposition and passivated hydrogen evolution in highly acidic sulphate electrolytes: depassitivation by nickel impurities", Journal of Applied Electrochemistry, 20 (1990), p. 1009-1014.; S. K. Gogia, S. C. Das, "The effect of Co, Cu, Fe and Fe during electrowining of nickel", Journal of Applied Electrochemistry, 21 (1991), p. 64-72!. However, there is no study on the effect of impurities on the newly developed nickel-zinc alloy electroplating A. Brenner, "Electrodeposition of Alloys", Two Volumes, Academic Press, Inc., New York (1963); R. Shula, S. K. Srivastava, and S. C. Srivastava, "The role of addition agents in the electrobath", Surface and Coating Technology, 28 (1986), p. 129-231!.